PYROMETRY 463 The thermometer is protected by a porcelain tube glazed on the outside, or by a quartz tube which fits into the terminal head. For industrial use the refractory tube is itself protected by an outer metal tube. The metal tube is usually fitted to an extra terminal head through which the lead wires to the line are carried. On account of the deterioration of the platinum, thermometers constructed of fine wire are not very satisfactory for industrial use above 900°C. Heavy platinum wire, 0.6 mm. in diameter, may be employed up to 1,100°C. but the resistance of the thermometer is so low that it is necessary to use methods of measurement which do not depend upon constancy in the resistance of the lead wires, e.g. Thomson bridge or potentiometer. In the Engelhard resistance thermometer a coil of fine platinum wire having a resistance of 50 ohms at 0°C. is wound upon a quartz rod 3 mm. in diameter and 6 cm. in length and fused inside a quartz tube so that the platinum is entirely embedded in quartz. Gold leads are carried from the coil to the terminal head. The quartz insures high electrical insulation while still preserving excellent heat conduction. The thermometer is compact and the platinum is thoroughly protected against deterioration. For work of the highest precision, in which measurements to 0.1° or 0.01° are required this type of thermometer is inferior to that in which the platinum is loosely mounted on a mica frame since its calibration is slightly altered with use on account of the strain introduced by the differential expansion of quartz and platinum. For most purposes this objection is of small consequence. The relation between the temperature t°C. and the resistance rt of the platinum resistance thermometer is of the following form where a, 6, and c are constants determinable by the standardization points. rt = a + It + d* If only two lead wires are carried from the, thermometer coil to the indicator, the resistance measured is the sum of the resistances of the coil, the platinum or gold lead wires to the head of the thermometer, and the copper lead wires from this point to the indicator. The resistance of the platinum or gold lead wires will depend upon the form of temperature gradient along the thermometer from the bulb to the head and upon the depth of immersion. Hence, this variable resistance is introduced into the temperature measurement, and changes in the resistance of the lead wires will be interpreted as changes in the temperature of the thermometer coil. There are three general methods for minimizing or eliminating the effect of variation in lead resistance. 1. Use of a coil of high resistance: This is the method employed in the Engelhard thermometer. The resistance of the lead wires from the coil to the thermometer head and the likely variation in resistance of the copper lead wires are extremely small compared to the 50-ohm platinum coil. Hence such variations in resistance as are likely to occur do not introduce serious error. Thus only two lead wires are necessary, from the thermometer to the indicating instrument. 2. Compensating lead wires of the Callendar or Siemens form: Both of these methods of compensation are applicable to measurements made with the Wheatstone bridge. The Callendar form of compensation requires four lead wires. Two of the lead wires joined to each end of the coil are connected in one arm of the bridge and the other two lead wires, which are "dummy'7 leads formed of a single loop of wire extending to the top of the coil, are connected in the corresponding arm of the bridge. The two sets of lead wires are exactly alike so that variations in temperature affect each set similarly. The Siemens method of compensation described below requires three leads and is more often employed industrially in this country. 3. Potential terminals: The use of potential terminals is necessary when the resistance of the thermometer is made very low or when the precision required is such as to make variations in difference of lead resistances inadmissible.